Electrochemical reactors are known as such, they usually comprise one or more electrically connected electrochemical single cells, arranged in a bipolar arrangement or a unipolar arrangement, the latter is also called mono-polar arrangement.
A bipolar arrangement finds frequent use in solid polymer electrolyte technologies, for example PEMFC and PEM electrolysers. In an electrochemical cell stack with bipolar arrangement, the so-called bipolar plate is a conductive element placed between two cells. It connects electrically adjacent cells in series (“Encyclopedia of Electrochemical Power Sources”, ISBN: 978-0-444-52745-5) Electrons generated or consumed within the active layer of the electrode flow in a direction perpendicular to the plane of the electrode (y-axis), and pass through the bipolar plate, situated between the anode of one cell and the cathode of the adjacent cell, they do not have to flow in the plane of the electrode to the collection point(s) at the edge of the electrode (lug).
In unipolar arrangement, the electrochemical cells forming the stack/electrochemical reactor are externally connected. In an electrochemical reactor/stack of galvanic cells with unipolar arrangement the anode of a cell is electrically connected to the cathode of an adjacent cell using an external electrically conductive element, e.g. a cable, metal wire, etc., which is attached to a current feeder bar mounted along an edge of the electrode, or along part of that edge. The electrons are collected by the current feeder at the edge of the anode, an external cable connects the anode to the cathode of the adjacent cell (“Fuel cells: fundamentals and applications”, ISBN 978-0-387-35402-6).
In an electrolyser with a unipolar arrangement, a plurality of alternating positive and negative electrodes forming a stack are separated by ion permeable membranes. In an electrolyser with unipolar design, electrochemical cells forming the stack are externally connected, the positive electrodes are electrically connected in parallel as well as the negative electrodes. The assembly is immersed in an electrolyte bath or tank. A cell stack with unipolar arrangement requires collection of the electrons at the anode and an external connection to the cathode of the next cell. Unipolar arrangements of electrochemical cells find widespread use in low power applications and special applications where replacement of a malfunctioning single cell may be required during operation. Unipolar arrangement namely permits simple and easy identification and replacement of malfunctioning cells, which is not the case in a bipolar stack (“Fuel Cell Science and Engineering: Materials, Processes, Systems and Technology”, ISBN: 9783527650248). Unipolar arrangement is also preferred in the case of electrochemical cells with liquid electrolyte in which direct contact between anode and cathode is prevented by the presence of an inert spacer material. However, care needs to be taken to select an appropriate spacer material, as it may unnecessarily increase the weight and the dimensions of the cell.
Major limitations of unipolar arrangements are their relatively high cost and the fact that the power density distribution over the current density distributor may be irregular, as a result of which the power density may be locally insufficient. In a unipolar arrangement the electron current is transported over the length of the electrode between opposite sides of the electrode over the plane of the electrode, which connect a current feed and a current collector of the electrode. It has been observed that not all of the current seems able to flow over the entire electrode surface and reach the edge along which the current is collected. As a result, local ohmic over-potential can be relatively high, which hampers the efficiency of the electrolytic cell.
The problem that conventional porous electrodes and gas diffusion electrodes do not show a sufficient internal conductivity to permit collecting a major part of the electrons at the current collector at the edge of the electrode, has been solved by the incorporation of a current density distributor which is conductive to a major part of the electrode structure. Frequently used current density distributors comprise a metal mesh, which is incorporated into the active, porous layer of the electrode, where the electrochemical reaction is carried out. The mesh adds the required in-plane conductivity to the electrode in the direction along the direction of major current flow and in cross direction thereof and provides mechanical and dimensional support to the electrochemically active layer of the electrode. Metal grids or meshes with a low electrical resistance made of a variety of alloys in a wide combination of thicknesses and open areas, are commercially available.
However, the existing metal mesh current density distributors, in particular when used in unipolar electrodes, present several disadvantages. The metal wires are quite expensive and thereby significantly contribute to the cost of the electrode. Besides this, the metal wires have a high density, as a result of which the weight of the electrode may raise quite high if a certain current carrying capacity is envisaged (see “Fuel cells: fundamentals and applications, ISBN 978-0-387-35402-6, “Encyclopedia of Electrochemical Power Sources”, ISBN: 978-0-444-52745-5, “Fuel Cell Science and Engineering: Materials, Processes, Systems and Technology”, ISBN: 9783527650248.)
EP0.051.437 discloses an electrolytic cell which is used for the production of chlorine gas and sodium hydroxide from a saturated sodium chloride salt solution. Although the use of oxygen (air) cathodes permits suppressing undesired formation of molecular hydrogen at the cathode, molecular hydrogen formation still accounts for approximately 25% of the electrical energy consumption used to operate the cell.
The oxygen cathode disclosed in EP0.051.437 comprises an active layer of silver catalyzed active carbon particles positioned within an unsintered network of fibrillated carbon black-polytetrafluoroethylene. The “working” face of the active layer is covered with an asymmetric woven wire mesh current density distributor, the other opposite face of the active layer is covered with a layer of a porous, wet-proofing backing material for example made of PTFE. The asymmetric woven wire mesh current density distributor has been designed in such a way that it has more conductive wires in the direction generally perpendicular to the major current feed to the current density distributor than in the direction generally parallel to the direction of major current feed. The generally perpendicular wires span the narrow (shorter) conductive path of the electrode. The asymmetric woven wire mesh current density distributor preferably has from 1.5 to 3 times as many such perpendicular wires as parallel wires, in particular 50 strands/inch of perpendicular wires and 25 strands/inch of parallel wires versus conventional symmetrical woven wire mesh having a wire thickness of 0.005 inch (0.127 mm). It is explained that due to the asymmetric structure of the wire mesh current density distributor, substantial economies in material and weaving costs can be achieved, while an efficient current distribution and control of the direction of current travel with its resulting control of current path, can be achieved.
The current density distributor disclosed in EP0.051.437 however presents the disadvantage that only a limited portion of the wires in perpendicular direction may be removed, if mechanical and dimensional stability are to be guaranteed as well as a sufficient support to the active layer. Thus according to EP0.051.437 a limited weight and cost reduction may be achieved only.